This application is based on Japanese Patent Application Nos. 2000-399199 filed Dec. 27, 2000, and 2001-290782 filed Sep. 25, 2001, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates in general to a method of producing fine particles of a metal oxide having a nanoscale grain size.
2. Discussion of Related Art
In the field of electronic ceramics, it is desired to reduce the grain size of the materials of the electronic ceramics. In the field of a gas sensor, for instance, it is expected to improve its sensitivity and lower its operating temperature, by reducing the grain size of the material down to the order of several tens of nanometers (nm) for thereby reducing the crystal grain size of the material. In the electronic ceramics such as a varistor and a thermistor which utilize the characteristics of the crystal grain boundaries, the grain boundaries are increased and the characteristics of the electronic ceramics are improved by reducing the crystal grain size and densely sintering the crystal grains. Based on a fact that the reduction of the crystal grain size of a metal oxide results in a quantum effect, it is expected that the finely grained material whose grain size is reduced is used to provide new-type functional ceramics utilizing the quantum effect. Since the above-described effects will appreciably increase with a decrease of the crystal grain size of a sintered body of the metal oxide, it is desirable that the crystal grain size is reduced down to the nanometer scale, in other words, the sintered body of the metal oxide is constituted by the nanocrystal grains. The metal oxide is used as a material of a grinding stone, for instance. The sharpness of cutting edges provided by the abrasive grains increases with a decrease of the crystal grain size of the grinding stone, in other words, with a decrease of the size of the abrasive grains which constitute the grinding stone. When the metal oxide is used as free abrasive grains, the quality of the surface which is subjected to a finish grinding by the free abrasive grains can be increasingly enhanced by reducing the grain size of the metal oxide. It is noted that the term xe2x80x9cnanocrystal grainsxe2x80x9d refers to the crystal grains having a grain size of not greater than about 100 nm. A sintered body in which the nanocrystal grains are densely bonded together is refereed to as xe2x80x9ca dense sintered body of nanocrystal grainsxe2x80x9d.
As a method of producing a fine powder as described above, a chemical process using an inorganic salt is known, for instance. This method comprises the steps of: adding an alkaline solution to an aqueous solution of a metal chloride; precipitating a hydroxide of the metal; and heat-treating (calcining) the precipitated hydroxide in a non-reducing atmosphere after it has been dried. This method is described, for instance, in xe2x80x9cThe Stannic Oxide Gas Sensorxe2x80x9d (pp. 11-47) published by CRC Press in 1994. For obtaining a powder of a stannic oxide (tin oxide) used for fabricating a gas sensor, tin tetrachloride (SnCl4) is used as a starting material and an ammonia water is used as the alkaline solution. FIG. 1 shows the conventional process steps for producing the tin oxide. In this method, however, the precipitated hydroxide having a nanoscale grain size is aggregated into an agglomerate after drying, and the nanometer-sized primary grains are fixedly bonded together to form coarse grains (secondary grains) after calcining. Accordingly, even if the product is mechanically pulverized into a powder each time after drying and calcining, the size of the secondary grains obtained by the mechanical pulverization is inevitably greater than about several microns (xcexcm). Further, the purity of the pulverized powder is undesirably low since impurities get into the powder during the pulverizing operation.
For the purpose of obtaining the fine and highly pure powder of a metal oxide, various methods are proposed. For instance, JP-A-7-187668 proposes a method of oxidizing a graphite intercalation compound obtained by reacting a compound of a specific metal (such as nitrate or oxynitrate) with a graphitic carbon modification. The fine powder of the metal oxide having a high degree of purity is also produced by hydrolysis of an alkoxide material. Further, J. Aerosol Sci., Vol 24 (pp.315-338) published in 1993 describes nanostructured oxides synthesized by thermal vaporization/magnetron sputtering and gas condensation, wherein a metal is subjected to an oxidizing treatment after it has been evaporated and deposited in a vacuum. All of these methods, however, have disadvantages described below. In the method using the graphitic carbon modification, the metal to be used is limited to the one which is capable of forming the graphite intercalation compound. Further, it takes a relatively long period of time to form the graphite intercalation compound, deteriorating the production efficiency. In the method using the alkoxide, the material (alkoxide) is rather expensive for mass-production of the fine powder of the metal oxide. In the gas-phase evaporation method, the rate of formation of the fine power of the metal oxide is considerably low, deteriorating the production efficiency.
Accordingly, the conventional methods described above experience difficulty in mass-producing a fine powder of a metal oxide, making it difficult to use the fine powder of the metal oxide for various applications which require the fine powder. Further, it is difficult to mass-produce a sintered body formed of considerably fine crystals whose grain size is not greater than several tens of nanometers (nm).
The present invention was developed in the light of the background art described above. It is therefore an object of the invention to provide a method of producing fine particles of a metal oxide having a grain size on the order of nanometer (nm).
The object indicated above may be achieved according to an aspect of the present invention, which provides a method of producing fine particles of an oxide of a metal, comprising the steps of: preparing an acidic solution which contains ions of the metal; precipitating fine particles of a hydroxide of the metal by adding an alkaline solution to the acidic solution; collecting the fine particles of the hydroxide of the metal precipitated in a mixed solution of the acidic solution and the alkaline solution; mixing fine particles of a carbon with the collected fine particles of the hydroxide of the metal; and heat-treating a mixture of the fine particles of the hydroxide of the metal and the fine particles of the carbon at a predetermined temperature in a non-reducing atmosphere, whereby the fine particles of the oxide of the metal are produced.
According to the present method described above, the fine particles of the hydroxide of the metal (the metal hydroxide) which have been precipitated in the mixed solution of the acidic solution and the alkaline solution and collected therefrom are mixed with the fine particles of the carbon, and heat-treated in the non-reducing atmosphere, so that the fine particles of the oxide of the metal (the metal oxide) are produced. In the present method, the fine particles of the carbon are mixed with the fine particles of the metal hydroxide prior to the heat-treatment including the drying and calcining steps in the synthesis of the fine particles of the metal oxide according to the above-described chemical process using the inorganic salt. According to the present method, the fine particles of the carbon which are mixed with the fine particles of the metal hydroxide are effective to prevent the formation of the coarse secondary grains in which the fine particles of the metal hydroxide are bonded together. Accordingly, the present method provides the fine particles of the metal oxide having a nanoscale grain size owing to or derived from the nanoscale grain size of the precipitated metal hydroxide. In the present method described above, the process steps for producing the fine particles of the metal oxide are not complicated as compared with the conventional methods. Further, the present method provides the fine powder of the metal oxide by a simple aqueous synthesis without requiring any special material and equipment, permitting the mass-production of the fine powder of the metal oxide at a relatively low cost without adversely influencing the environment. Thus, the method according to the present invention permits the mass-production of the fine particles of the metal oxide whose grain size is on the order of nanometer (nm). In the conventional chemical process using the inorganic salt described above, the metal chloride used as the starting material is dissolved in a solvent, for thereby preparing the acidic solution. In the present invention wherein the neutralization reaction of the acid and the alkali is utilized to precipitate the metal hydroxide, the acidic solution may be prepared in any manners provided that the prepared acidic solution contains the ions of a specific metal whose oxide is to be obtained by the present method.
For the following reasons, the fine particles of the carbon prevent the fine particles of the metal hydroxide from being bonded together, and accordingly contribute to the formation of the fine powder of the metal oxide. In the heat treatment effected in the conventional method of producing the powder, the primary grains having the nanoscale grain size are bonded together, so that the coarse secondary grains are inevitably formed. If the fine particles of the carbon are mixed with the metal hydroxide, the fine particles of the carbon are heated during the heat treatment, and oxidized into a carbon dioxide gas having a high pressure within the agglomerate of the metal hydroxide. Accordingly, the secondary grains in the dry state are pulverized into the primary grains having the grain size of several tens of nanometers (nm) by the generated gas pressure. The thus obtained primary grains are oxidized into the fine particles of the metal oxide. Since the fine particles of the carbon are burnt out, the fine particles of the carbon which have been added to the metal hydroxide do not remain within the obtained fine particles of the metal oxide, avoiding a risk of deteriorating the purity of the fine particles of the metal oxide.
In one preferred form of the present invention, the fine particles of the carbon are mixed with the fine particles of the hydroxide of the metal in a proportion of not less than 1.5% by mass with respect to the fine particles of the hydroxide of the metal. According to this arrangement, the ratio of mixing the fine particles of the carbon with the metal hydroxide is suitably determined, and the agglomerate of the metal hydroxide can be effectively pulverized into the primary grains. Even when the amount of the fine particles of the carbon to be mixed is larger than required for pulverizing the secondary grains into the primary grains, it is possible to obtain the fine particles of the metal oxide having the nanoscale grain size although the excessive amount of the fine particles of the carbon is wasted. In essence, the upper limit of the amount of the fine particles of the carbon is not particularly limited, provided that the mixing ratio or proportion of the fine particles of the carbon with respect to the metal hydroxide assures that the entire amount of the secondary grains of the metal hydroxide are pulverized into the primary grains.
In another preferred form of the present invention, the fine particles of the carbon have a primary grain size of 1-50 nm. According to this arrangement, the grain size of the fine particles of the carbon is sufficiently small, assuring a high degree of dispersion thereof in the fine particles of the metal hydroxide. Therefore, the entire amount of the secondary grains of the metal hydroxide can be efficiently pulverized into the primary grains by addition of a relatively small amount of the fine particles of the carbon. If the primary grain size of the fine particles of the carbon is less than 1 nm, the fine particles of the carbon are not likely to be uniformly mixed with the metal hydroxide due to aggregation or cohesion, for instance.
In still another preferred form of the present invention, the fine particles of the carbon has a turbostratic structure. The fine particles of the carbon having the turbostratic structure exhibit a high degree of fluidity, for thereby assuring a high degree of dispersion in the metal hydroxide.
In yet another preferred form of the present invention, the metal consists of at least one element selected from the group consisting of silicon, manganese, zirconium, chromium, iron, nickel, tin, zinc, indium, aluminum, cerium, magnesium, and titanium. Since each of the hydroxides of these elements is insoluble in water, the precipitated hydroxide can be easily collected, permitting easy production of the oxide. Each of the oxides of tin, zinc, and indium functions as a conductive material, and is effective to improve the performance and reduce the cost of the parts which are required to exhibit a high degree of conductivity while assuring a high degree of production efficiency. Each of the oxides of silicon, manganese, zirconium, chromium, iron, nickel, aluminum, cerium, magnesium, and titanium functions as an insulating material, and can be also used as a material of the grinding stones, assuring a high degree of performance and production efficiency in those applications.
In a further preferred form of the present invention, the step of preparing the acidic solution comprises a step of dissolving a salt of the metal in a solvent. According to this arrangement, the acidic solution which contains the metal ion can be easily prepared by dissolving the salt of the specific metal in a suitable solvent. More preferably, the salt of the metal is selected from the group consisting of nitrate, carbonate, sulfate, acetate, and chloride.
In a still further preferred form of the present invention, the alkaline solution is an ammonia water.
In a yet further preferred form of the present invention, the predetermined temperature at which the step of heat-treating the mixture is effected is selected within a range of 500-1000xc2x0 C.